During my junior year, my grades slipped drastically. 1st semester, 3 B's out of 8, 2nd semester, 1B and 1 C out of another 8 classes. The C I might be able to move up I found out. However, I had a health problem during this year so missed school and stuff. It wasn't anything huge like missing 3 monthes, more like a day here and there. Turns out, I was allergic to something--yeah i know, really stupid. Tell the docs that. Would this be a valid excuse for low grades ? I know colleges like to see an upward spiral for grades.

If you're looking at the elites, your excuse won't help you. They're accustomed to applicants who overcame HUGE obstacles and still managed to do well in school. My feeling is that if you mention this, hoping to explain away your drop in grades, they'll view you as a whiner.

Here's a bigger problem - many of the top colleges have very, very old buildings that haven't been renovated in a while. Residing and spending time in those old, moldy buildings can hurt an allergic person pretty badly.